1. Field of the Invention
The present invention relates to a control circuit for operating single-speed and two-speed three-phase motors from single-phase current.
2. Description of the Prior Art
Control circuits for starting and operating three-phase motors, including single-speed and two-speed motors are known in the art. All of these known techniques of applying static phase converters have heretofore employed a feedback voltage or current derived from the motor winding or windings or motor control. By this exact nature, the motor winding characteristics and, therefore, the horsepower must be exactly matched to the control circuit for proper and reliable operation.
Typically, the starting torque is applied to only one of the three motor windings and, after achieving synchronous speed, is disconnected from that winding to run on a single phase source only one two windings in the run mode. Synchronous speed, as is well known, is that speed which is the designed operating speed of the motor, whereby the rotor attains the speed and locks thereto. The disconnection of the starting torque current from the motor winding results in an obvious one-third reduction in horsepower. Inasmuch as the motor will be used in the manner and purpose for which it was designed, the loss of one winding in the run mode produces overheating and high noise levels due to the imbalance of winding currents. Disconnection is accomplished by way of a sensing circuit which constitutes a relay utilizing the back EMF from the motor windings to operate the voltage- or current- sensing relay.
It is unlikely, under normal operating conditions, once the rotor has attained synchronous speed, sufficient load would be applied to the rotor to cause a complete breakaway from synchronous speed such that the starting torque previously applied would be required to restore the rotor back to synchronous speed. In the case of extreme and excessive load conditions, the characteristics of the motor are such that an increased slip develops between the rotor and field windings and the load current increases to such an extent that thermal protective devices open and disconnect the motor from the power supply. Therefore, it is not practical to attempt to reapply starting torque during these periods of excessive and inordinate load conditions. It is necessary, however, that during start-up of a motor subjected to reasonable starting loads, such as air compressors, gear boxes, mixing machines and the like, that the starting torque be applied for as long a period as necessary to overcome these excessive starting loads and thereby attain the required synchronous speed.
If the relay operates too soon, the motor will not achieve full or synchronous speed. If the relay operates too late, the intermittent duty electrolytic capacitor and the motor windings will be damaged by excessive starting current.
It is obvious to those skilled in the art that this type of phase conversion must be designed and adjusted for each different horsepower rating, resulting in many different models.
Also, deficient in previous structures is the ability to operate two-speed motors from a single control circuit. The use of two-speed motors has increased dramatically, particularly in the application of lathes and milling machines for which the available working speeds can be coupled simply by switching from low-speed winding connections to high-speed winding connections. Again, a feedback voltage or current is required from either of the two separate windings to operate a relay to disconnect the starting torque at, typically, 70% of synchronous speed. The difference in motor winding impedance produces voltage and current variations too great for a single coil relay to operate at the conventional 70-80% of synchronous speed, for both high and low horsepower and speed ratings.